1. Field of the Invention
The present invention relates to a method for forming a dielectric film, and more particularly, to a method for forming a high-permittivity dielectric film for use in a semiconductor device.
2. Description of the Prior Art
As the overall dimensions of semiconductor devices continue to shrink, the demand is ever increasing for devices having large charge storage capacity. The need for large charge storage capacity remains even though individual components are scaled to smaller dimensions. As the surface area of a component, such as a capacitor, is reduced, a corresponding reduction in charge storage capability occurs. The smaller surface area available for components, such as transistors, capacitors, and the like, coupled with the requirement to maintain high charge storage levels, has led researchers in the field of fabrication science to seek new materials from which to construct the components. One group of promising new dielectric materials is the family of high-permittivity ferroelectric materials such as tantalum pentoxide (Ta.sub.2 O.sub.5), BST ((Ba,Sr)TiO.sub.3), or PZT (Pb(Zr.sub.1-x Ti.sub.x)O.sub.3).
While the ferroelectric materials offer a substantial improvement in compact charge storage capability, the use of the ferroelectric components in MOS integrated circuit technology has been limited by the physical and chemical characteristics of the ferroelectric materials. For example, stoichiometrically the Ta.sub.2 O.sub.5 film frequently runs short of oxygen, resulting in permittivity degradation and undesired leakage current characteristics. Moreover, the Ta.sub.2 O.sub.5 film has a bad interface characteristic with a polysilicon film or a metal nitride film commonly used as an upper electrode, and is in a high intrinsic stress state, thus remaining many problems to be solved.
In case of using a Ta.sub.2 O.sub.5 film as a capacitor dielectric film according to the prior art techniques, the stoichiometrical shortage of oxygen becomes severe by residual gases or byproducts, resulting in undesired leakage current increase. Additionally, if post-thermal treatment is performed in an oxygen atmosphere and at a temperature of 800.degree. C. or more after the formation of the Ta.sub.2 O.sub.5 film, gases are overflowed, and thus an amorphous Ta.sub.2 O.sub.5 film is crystallized into a columnar structure. At this time, oxygen diffuses quickly along grain boundaries, and a silicon oxynitride (SiON) film is thus thickly formed between a polysilicon layer and a Ta.sub.2 O.sub.5 film in the case of using a polysilicon layer as a lower electrode. This adversely effects the realization of high capacitance promised by the Ta.sub.2 O.sub.5 film. In the case of using a metal nitride film as an upper electrode of the capacitor, an interface reaction occurs between the Ta.sub.2 O.sub.5 film and the metal nitride film, so that the electrical characteristics of the capacitor deteriorates due to poor adhesion therebetween.
Accordingly, the present inventor proposed a method for fabricating a semiconductor device using tantalum oxynitride films instead of Ta.sub.2 O.sub.5 films in the present inventor's co-pending Korean patent applications 98-31766 and 98-32106. However, the techniques disclosed in the prior applications include forming a tantalum oxynitride film directly from the reaction between the respective gases containing tantalum, nitrogen and oxygen, which are not compatible with the conventional processes for forming a tantalum oxide film or a tantalum nitride film. Thus, there is a need for seeking optimized process conditions to form a tantalum oxynitride film directly.